It's not uncommon. Sometimes it's caused by a corrosive adhesive on cheaper grade of electrical tape. Some tape adhesives react corrosively with WD-40.

In my early years of knifemaking I had a lot of this type of thing happen. Personally I never cover a blade anymore..... I've found it's best to just be cautious and take care not to damage a finished blade, versus dealing with the problems caused by attempting to protect a blade.

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Caffreyknives@gmail.com"Nobody cares what you know.....until they know you care."See me at table 2Q at the Blade Show!

Apparently Ed has heard of using electrical tape for this purpose before but that's a new one for me. When the subject of taping a blade comes up 3M blue masking tape seems to be what most people use, it certainly is my choice. I've never had a problem with it. I'd go so far as to say that, for me at least, it would be 3M blue or nothing at all ....

Never much liked using electrical or duct tape on metals. Both types always seem to leave a pretty nasty residue or discoloration on the blade, never had it etch though. I like masking tape. Generous coating of oil on the blade, good wrap of masking tape. Protects it from scratches when putting the handles on and come off clean

Same thing here a heavy tape like duct tape electrical tape has a lot of adheasive on it....that you would need to clean off with something like acetone. with masking tape there is no where near as much adheasive most of the time it comes off clean but sometimes if you leave it on for a while or you really press it on the the blade it might leave a LITTLE adhesive a lil acetone on a paper towel or micro fiber cloth it wipes right of in one or 2 swipes something like duct tape you need to do more scrubbing to get it off OR just put the blade in a bucket of acetone...a minute or 2 it should wipe off very easily but if you let it soak for a while you wont even need to wipe it off the acetone will dissolve the adheasive after a while